Update FAQ with more common questions

readthedocs/quick-references/faq.rst

@@ -198,6 +198,23 @@ won't do unnecessary work unless you need to:
         sender = await event.get_sender()
 
 
+File download is slow or sending files takes too long
+=====================================================
+
+The communication with Telegram is encrypted. Encryption requires a lot of
+math, and doing it in pure Python is very slow. ``cryptg`` is a library which
+containns the encryption functions used by Telethon. If it is installed (via
+``pip install cryptg``), it will automatically be used and should provide
+a considerable speed boost. You can know whether it's used by configuring
+``logging`` (at ``INFO`` level or lower) *before* importing ``telethon``.
+
+Note that the library does *not* download or upload files in parallel, which
+can also help with the speed of downloading or uploading a single file. There
+are snippets online implementing that. The reason why this is not built-in
+is because the limiting factor in the long run are ``FloodWaitError``, and
+using parallel download or uploads only makes them occur sooner.
+
+
 What does "Server sent a very new message with ID" mean?
 ========================================================
 
@@ -261,6 +278,17 @@ same session anywhere else. If you need to use the same account from
 multiple places, login and use a different session for each place you need.
 
 
+What does "Task was destroyed but it is pending" mean?
+======================================================
+
+Your script likely finished abruptly, the ``asyncio`` event loop got
+destroyed, and the library did not get a chance to properly close the
+connection and close the session.
+
+Make sure you're either using the context manager for the client or always
+call ``await client.disconnect()`` (by e.g. using a ``try/finally``).
+
+
 What does "The asyncio event loop must not change after connection" mean?
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